The present invention relates to bipolar semiconductor devices, particularly but not exclusively where a bipolar semiconductor device is required to switch a high power load at high speed, having a semiconductor body, a first region of that body forming an emitter region, a second region of that body forming a base region and means for promoting the recombination of carriers in the base region of the device, and further relates to a method of manufacturing such a semiconductor device.
It is known that the operation of bipolar semiconductor devices, such as power transistors, is impeded by the occurrence of a phenomenon known as `second breakdown`. This phenomemon is connected with a transient flow of current during the turn-off period of the transistor through a part of the transistor structure which is located centrally with respect to the emitter region and which may extend through the base region and the collector region of the transistor. Second breakdown occurs when local concentrations of the transient current occur and local heating creates hot spots which generate carriers by thermal means. Thermal runaway then occurs which eventually leads to the destruction of the transistor.
Control of the transient current may be exercised by providing carrier recombination centers in the base region and the collector region of the transistor. Carrier recombination centers may be provided by diffusing gold or platinum metal atoms into the base region and collector region of the transistor. The deep level carrier recombination centers so produced reduce the lifetime of carriers in the said regions by an amount dependent on the concentration of the gold or platinum atoms. Unfortunately, both gold and platinum diffuse rapidly in silicon, so the carrier recombination centers are distributed throughout the base region and collector region and although effective in combatting second breakdown, the presence of the carrier recombination centers impedes the flow of current from the active emitter region edges to the collector region when the transistor is on.
In addition to impeding the flow of current the presence of the metal atoms may cause excess leakage to occur at the boundary between doped regions of semiconductor devices. The excess leakage is caused by the precipitation of the metal atoms at these boundaries.
The problems described above inherent in the use of gold or platinum or other metal atoms, for example silver and copper, to provide deep level carrier recombination centers are recognized in U.K. Patent Specification No. 1511012 (General Electric Company). To overcome these problems the semiconductor device structure disclosed is one in which carrier recombination centers, provided by metal atoms, are confined in columnar arrays within the device structure. Each column is formed of a region of recrystallized material extending perpendicularly into the body of the semiconductor from a major surface and intersects a p-n junction at a part of the p-n junction which lies parallel to the major surface of the body. Each recrystallized region extends through at least one layer of a first conductivity type of the structure and extends at least part way through an adjacent layer of a second conductivity type of the structure.
A bipolar semiconductor device according to the invention of U.K. Pat. No. 1511012 thus has a semiconductor body with a first region of that body forming an emitter region and a second region of that body forming a base region. The part of the emitter region-base region p-n junction which lies parallel to a major surface of the body is substantially that part of the junction between the edges of the emitter region which are active when the device is on. The active zone of the base region lies substantially beneath the part of the emitter region-base region junction which is parallel to the major surface of the semiconductor body. The higher concentration of carrier recombination centers is within the active zone of the base region and extends through the emitter region and at least part way through the adjacent base region.
The columnar array is provided by further processing a semiconductor body which has been processed in accordance with well-known techniques to provide within the semiconductor body a finished device structure having regions of a first and a second opposite conductivity type and p-n junctions between the regions lying substantially parallel to the major surfaces of the semiconductor body.
The further processing includes masking a major surface with an acid-resistant material, preferably SiO.sub.2, deposited by thermal oxidation or chemical vapour deposition, and then providing a photoresist mask over the acid resistant mask. The photoresist mask is then exposed and developed to form a mask through which the SiO.sub.2 is etched with buffered HF to give an array of holes through the SiO.sub.2 of not more than 10 micrometer (.mu.m) diameter not more than 15 .mu.m apart. After removal of the remaining photoresist, the surface of the semiconductor body is etched, using the previously-etched SiO.sub.2 layer as a mask, to form depressions in the surface. After a post-etch rinse and dry, the metal which is to provide the recombination centers is evaporated onto the remaining SiO.sub.2 layer and into the depressions in an evaporation chamber at a pressure of not more than 5.times.10.sup.-5 torr. A thermal migration process at a temp between 400.degree. and 1400.degree. C. with a temperature gradient of 50.degree. C./cm was then used to drive droplets formed of the metal evaporated into the depressions to a required depth within the structure, or through the full thickness of the structure. Where the droplet was only driven to a required depth, for example, part way through the base region of the device, a reverse thermal migration was used to drive the droplet back to its starting point at the surface depression. The major surface at which the droplet resided after thermal migration was then ground to remove the droplet and etched to remove the effect of grinding. If this major surface was other than the major surface from which the migration of the droplet commenced both major surfaces of the semiconductor body were ground and etched to leave them ready for the deposition of electrodes and other processing required to complete the semiconductor device.